You can associate address number nodes in a limited fashion today:

1.) Create a node
2.) Assign addr:housenumber to the node.
3.) Invoke the Address Interpolation plugin while selecting just the node and street. You can create the relation from there also if desired.

So - will it be worth the improvement to add a mode where selecting single blank node plus street will ask for the house number?

That will also work while selecting the street and multiple nodes that already have addr:housenumber assigned.

Adding a closed way (building) to the street ‘associated’ role of the relation begins to stray outside the focus of the plugin, and the ‘collected / associated’ street relations are still at the proposed stage, if I read correctly.